Thin rubber films, prepared by dipping shaped formers into a compounded latex and then heating to dry and vulcanise the latex deposit on the former, have been used for many years in applications for which their barrier properties have proved invaluable. Natural rubber latex has proved to be particularly useful in this respect since it gives films that are strong, highly deformable, and show good recovery on removal of stress. Such films have been used extensively for the manufacture of gloves for domestic, industrial, and surgical and general medical use, and for condoms and balloons. Particularly in medical applications, it is important that the film is impervious to blood, sperm, and other body fluids, and to micro-organisms, whilst at the same time being thin and flexible enough to allow sensitivity of touch and feel. Thin natural rubber latex films fulfil these requirements admirably and are in many ways ideal for such applications.
However, although these thin films are strong, they are subject to tearing and puncture by sharp objects; that is, if a defect is present or is formed in the film, it can act as an initiation site for tearing. Once initiated, a tear is likely to propagate readily, since the rubber in use is usually under strain. There is therefore a need to increase the resistance of a latex rubber film to tearing.
Many attempts have been made to improve the tear resistance of rubber films. One approach has been to incorporate into the rubber, very small particles of an immiscible, harder, less deformable material, capable of stopping or deflecting a growing tear. A convenient and effective way of achieving this is to mix the harder material, in the form of an aqueous dispersion, emulsion or second latex, with the rubber latex, prior to dipping the shaped former into it. After drying and vulcanisation, the film then contains an essentially uniform dispersion of the fine, particulate material. Such particles, usually 0.1 to 50 microns in diameter, are able to blunt and divert the tip of a growing tear and thus enhance tear strength. Among the materials first used for this purpose were polymer resins, for example, a hydrazine-formaldehyde resin formed in the latex in situ, and carboxylated synthetic rubber latex, such as carboxylated styrene-butadiene rubber (CSBR) or carboxylated acrylonitrile-butadiene rubber (CNBR) latex, which were added to the natural rubber latex. Carboxylated rubbers, polystyrene and copolymers of vinyl acetate have also been used, as have styrene-butadiene copolymers. U.K. patent specification no. 2,088,389 teaches the use of poly (vinyl chloride) (PVC) as an additive to prevulcanised natural rubber latex, and similar claims have also been made for PVC and its copolymers.
The above proposals all make use of finely divided organic polymers. An alternative approach uses fine-particle silica as the hard component of the film. Various types of silica may be used but the most effective are those of smallest primary particle size, such as the fumed silicas. Aqueous dispersions of hydrophilic fumed silica, marketed under the trade mark “Aerosil”, are known for improving tensile strength and tear resistance in articles made form natural and synthetic rubber latices (Technical Bulletin Pigments, No. 33; Degussa A G, Postfach 110533, D-6000 Frankfurt 11, Germany; December 1982; see also, H. Esser and G. Sinn (Kautschuk und Gummi, 1960, 13, WT126-132). A similar material, known under the trade mark Cab-O-Sil, has also been described as useful in latex films for increasing tear resistance.
In all previous approaches, including those involving silica, the reinforcing agent has been added in particulate form to the rubber latex. We have found that this leads to certain disadvantages, particularly when using the above commercially available forms of particulate or colloidal silica. In particular, particulate materials, especially colloidal silica, when added to the latex tend to reduce the stability of the latex. Agglomeration frequently occurs, leading to large particles which may be visible to the eye. Colloidal silica is also generally expensive, and can only be obtained in dilute form (typically a 10-15% solution). This leads to increased transport and storage costs, and is inconvenient for latex compounding as further processing may be required, or other suitable measures may need to be taken so as to be able to incorporate into the latex the colloidal silica in the form supplied.